1. Cross-Reference to Related Applications
This invention relates to controllers for controlling current in an inductive load, to integrated circuits or systems having such controllers and to corresponding methods.
2. Background and Relevant Art
It is known to control current in an inductive load (such as a coil, a solenoid and a motor winding) by means of Pulse Width Modulation (PWM) and driver transistors in a full-bridge configuration. Based on measurements of the coil current, a controller executes a decision process to determine when to switch the drivers. A coil current sensor circuit is used to measure the coil current and feed back a measure of the coil current. This value is compared to a reference signal indicating a desired current, to produce a current error signal. The controller alters the timing of the switching dynamically to minimize the error signal. Typically the drivers are arranged as an H bridge to enable a DC supply voltage to be switched to drive the coil alternately with positive and negative voltages. Such drivers have a number of driving modes as follows: forward (also known as charge mode, with a positive drive voltage), fast decay (with a negative drive voltage) and slow decay (also called freewheel, with no drive voltage, but with the coil short circuited to allow the induced current to flow and decay). These modes are switched at different times to provide pulse width modulated control. The waveform of each pulse can be varied by using a mixture of the three modes within a single cycle of the PWM. This is shown as a mixed decay mode.
FIGS. 1, 2 and 3 show a current flow through a known H bridge in forward mode, slow decay mode and fast decay mode respectively. The H bridge has transistors U1, L1 coupled in series between the supply lines Vm and PGND and transistors U2, L2 are likewise coupled in series. Parasitic diodes are shown with dotted lines across each transistor. The inductive load, e.g. a coil, is connected across the mid points of the transistor pairs UI-L1, U2-L2. In the forward mode shown in FIG. 1, transistors UI and L2 are on and transistors U2 and L1 are off, so the current flows from supply line Vm through transistor UI then the inductive load, here a coil, then transistor L2 and to supply line PGND. In the slow decay mode shown in FIG. 2, transistors UI and U2 are off and transistors LI and L2 are on so the current flows from the coil or inductive load through transistor L2 to supply line PGND, to transistor L1 and back to the coil or inductive load. In the fast decay mode shown in FIG. 3, transistors UI and L2 are off and transistors L1 and U2 are on so the current flows from supply line PGND to transistor LI to the coil or inductive load, to transistor U2, then to supply line Vm. Many different ways of controlling the switching of the drivers for switching the transistors UI, U2, L1, L2 to control the coil current are known.
For example, the switching can be carried out at a fixed frequency or variable frequency. One variable frequency method is called peak current control with fixed off time. This involves switching into freewheel or slow decay mode for a fixed time interval, and then driving the coil or inductive load until the current reaches the desired reference value. In this case the driving period varies and so the switching frequency varies. This is also called top sensing since switching occurs to make the top of the pulses match the desired current level. Another method is peak current control with fixed frequency. In this case, the freewheel time is not fixed, but the total of drive time and freewheel time is fixed, so the switching has a fixed frequency. It is also known to have mean value control to address the issue that peak control gives a mean output which is always lower than the reference current.
One known controller is the Allegro 3977 integrated circuit (IC). It uses a mixed decay mode with fixed off-time in its PWM current regulators, which limits the load current to a desired value. Initially, a diagonal pair of source and sink transistors are enabled and current flows through the inductive load as shown in FIG. 1. When the load current has the desired value, a current-sense comparator resets the PWM latch, which turns off both the source and the sink transistors in order to obtain mixed decay mode, and the current recirculates as in FIGS. 2 and 3. During this recirculation the current decreases until the fixed off-time expires. Mixed decay splits the fixed off-time of the PWM cycle into fast and then slow decay. After the fixed off-time of the PWM cycle, the appropriate transistors are enabled again, the inductive load current increases and the PWM cycle is repeated. Using mixed decay with fixed off-time has the advantage that PWM frequency is variable (lower peak in EMC spectrum, because energy in spectrum is smeared), but it needs relatively high frequency to guarantee operation above 20 kHz and this generates additional heat-losses. There are also external components involved, which adds to the cost and complexity. This prior art, which shows current-control with non-constant frequency PWM has the potential effect that PWM frequency could drop below 20 kHz (commonly used as an acceptable audible limit). The frequency variation is related to PWM generation of the current-control block that depends on coil-inductance, supply voltage, motor speed, current levels and other parameters. This can cause audible noise and related human discomfort.
Another known controller is the Infineon TLE-472x series ICs. These use a fixed frequency chopper with forward drive and brake (=slow decay) mode. Current-switching is top-sensing only. This known device apparently does not use a fast decay mode. Draw-backs of this include slow reaction time in certain motor load conditions and speeds: PWM duty cycle will be less than 50%.
Another known device is the Toshiba TB62200. This uses fixed frequency PWM with slow, fast and mixed decay. Using mixed decay mode, which requires additional switching points involves increased complexity. There is no indication of using other than top sensing.
U.S. Pat. No. 5,428,522 shows a four quadrant unipolar pulse width modulated (PWM) power conversion circuit for supplying a desired current to an inductive load such as a motor. The power conversion circuit uses an H-bridge circuit topology having an upper pair and lower pair of switching elements wherein the load is connected across a positive potential and negative potential DC power source. Diodes in parallel with each of the switching elements provide a current path from the load to the power source when its respective switching element is non-conductive. The value of the load current is compared to a desired load current value and switching element control signals are generated in accordance with a control algorithm to cause the instantaneous voltage across the load to alternate between a single polarity voltage and zero for a portion of the output load waveform to cause the average value of the load current to correspond generally with the desired average load current.
U.S. Pat. No. 4,757,241 is concerned with the problem that PWM systems which regulate current have required continuous monitoring of load current to avoid uncontrolled high frequency switching, or have exhibited discontinuity in control output when load current approaches the regulated value, or have not provided for a smooth transition from a current control mode to a voltage control mode. Two known methods to address this employ either a free-running oscillator to establish a fixed maximum frequency of operation or a monostable timer to establish a fixed off time. Each of these circuits have their advantages and disadvantages. To overcome the disadvantages, a logic means limits the cycling of the PWM enable signal to once per clock interval, and if desired, the logic means can establish a minimum time period during each clock interval in which the PWM enable signal may be inhibited in order to provide a minimum OFF interval for current decay in each cycle.